In an actually slightly related vein I have just treated a couple of our cars with Carlack68 glass sealant.
Seems to shed water like the proverbial ducks back, but since putting it on it hasn't rained so I haven't been able to try it in anger yet. Reports on detailingworld suggest it's good stuff though.
Gone: Fiesta - Focus - E46 320dSE - E91 330dMSport - E82 135iMSport - R55 JCW Clubman
Now: M135i with lots of toys